BiOBr/Bi2MoO6 composite in flower-like microspheres with enhanced photocatalytic activity under visible-light irradiation

Article abstract of DOI:10.1039/c5ra18987j

BiOBr/Bi₂MoO₆ composites in flower-like microspheres were synthesized by a facile solvothermal method. This photocatalyst could be activated by visible light owing to the narrow energy band gaps of both Bi₂MoO₆ and BiOBr. The BiOBr/Bi₂MoO₆ composites exhibited high activity in the photocatalytic degradation of rhodamine B and other organic pollutants under visible-light irradiation owing to enhanced light harvesting via multiple reflections inside the flower-like structure and due to the reduced photoelectron-hole recombination due to the formation of BiOBr-Bi₂MoO₆ heterojunctions. The dominant active species during the photocatalytic oxidation of RhB were determined as photogenerated holes and ·O₂^- was distinguished as the sub-main active site. The RhB mineralized into CO₂ through the de-ethylation route via forming five intermediates.

Full text of DOI:10.1039/c5ra18987j

RSC Advances
View Article Online
View Journal | View Issue
PAPER
BiOBr/Bi2MoO6 composite in flower-like
microspheres with enhanced photocatalytic
Cite this: RSC Adv., 2016, 6, 13498
activity under visible-light irradiation†
a
a
a
a
Yingchun Miao,ab Haibo Yin, Lu Peng, Yuning Huo and Hexing Li
*
*
BiOBr/Bi2MoO6 composites in flower-like microspheres were synthesized by a facile solvothermal method.
This photocatalyst could be activated by visible light owing to the narrow energy band gaps of both
Bi2MoO6 and BiOBr. The BiOBr/Bi2MoO6 composites exhibited high activity in the photocatalytic
degradation of rhodamine B and other organic pollutants under visible-light irradiation owing to
enhanced light harvesting via multiple reflections inside the flower-like structure and due to the reduced
photoelectron–hole recombination due to the formation of BiOBr–Bi2MoO6 heterojunctions. The
dominant active species during the photocatalytic oxidation of RhB were determined as photogenerated
Received 15th September 2015
Accepted 12th January 2016
DOI: 10.1039/c5ra18987j
ꢀ
holes and $O2 was distinguished as the sub-main active site. The RhB mineralized into CO2 through the
www.rsc.org/advances
de-ethylation route via forming five intermediates.
BiOBr crystal with a tetragonal matlockite structure is
composed of [Bi2O2]2+ slabs interleaved by double slabs of Br
Introduction
atoms. The layered BiOBr structure endows high carrier
mobility and small probability of the recombination of photo-
generated electrons and holes.15,16 We found that well-matched
band potentials in BiOBr/Bi2MoO6 heterojunctions could be
obtained, since the wider Eg and low CB potential in BiOBr
compared to those in Bi2MoO6 could facilitate the transfer of
photogenerated electrons from Bi2MoO6 to the CB of BiOBr,
leading to efficient charge separation and high photocatalytic
activity.
Herein, we fabricated BiOBr/Bi2MoO6 ower-like micro-
spheres with a uniform BiOBr/Bi2MoO6 combination by a facile
solvothermal route. The microspheres exhibited high activity in
the photocatalytic degradation of organic pollutants under
visible-light irradiation owing to the narrow energy band gap
and unique morphology and due to the BiOBr–Bi2MoO6 heter-
ojunctions. Meanwhile, the BiOBr/Bi2MoO6 combination dis-
played an unusual photocatalysis process.
Bismuth-containing semiconductors represent a new class of
promising photocatalysts useful for environmental cleaning
and H2 production, owing to their narrow energy band gaps for
absorbing visible light, and their hierarchical structure and
morphology as well as excellent stability.1,2 g-Bi2MoO6 has
received increasing interest due to its layered Aurivillius struc-
ture, in which perovskite slabs of corner-sharing distorted
+
MoO6 octahedra are sandwiched between (Bi2O2)2 layers,3,4
leading to the fast separation of photo-induced electrons from
holes and thus, a high quantum efficiency of photocatalysis.5,6
To further improve its photocatalytic activity, both nano-
structure engineering and composition doping have been
explored.7 The combination of two semiconductors allows the
transfer of photo-induced electrons from the valence band (VB)
of one semiconductor to the conduction band (CB) of the other
semiconductor, which could efficiently inhibit photoelectron–
hole recombination. Up to now, Bi2MoO6-based composites
with Bi2O3,8 ZnTiO3,9 Bi2WO6,10 TiO2,11 Bi2O2CO3,12 graphene13
and C60 (ref. 14) have been reported. However, their photo-
catalytic activities are still limited by the not well-matched band
potentials to a certain extent.5,6
Results and discussion
Structure characteristics
EDX analysis (Fig. S1†) demonstrated that only Bi, O, Br and Mo
existed in a 0.20-BiOBr/Bi2MoO6 sample. As shown in Table 1,
aThe Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of the ICP analysis revealed that the BiOBr content increased
RE Functional Materials, Shanghai Normal University, Shanghai 200234, China.
gradually in X-BiOBr/Bi2MoO6 with the increase of X. From
Fig. 1, the XRD patterns of X-BiOBr/Bi2MoO6 samples (X < 1.5)
E-mail: Hexing-li@shnu.edu.cn; huoyuning@shnu.edu.cn; Fax: +86-21-6432-2272;
Tel: +86-21-6432-1673
only show the diffraction peaks indicative of rhombohedral
Bi2MoO6 (JSPDS 21-0102), possibly due to the low content and
high dispersion of BiOBr. With the increase of BiOBr content,
the diffraction peaks at (011), (012) and (212) characteristic of
bFaculty of Chemical and Engineering, Key Laboratory of Environment Chemistry,
Qujing Normal University, Qujing 655000, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra18987j
13498 | RSC Adv., 2016, 6, 13498–13504
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Table 1 Structural parameters of different catalysts
XPS
ICP
Sample
mass (g)
Bi concentration
Mo concentration
Bi : Br :
Mo molar ratio
Bi : Br : Mo
molar ratio
SBET
RhB adsorption
rate (%)
Sample
(mg Lꢀ1
)
(mg Lꢀ1
)
(m2 gꢀ1
)
Bi2MoO6
—
—
29
35
31
33
—
—
64
76
65
64
—
—
—
19
19
19
21
29
35
8.7
15
17
18
21
0.10-BiOBr/Bi2MoO6
0.20-BiOBr/Bi2MoO6
0.33-BiOBr/Bi2MoO6
0.50-BiOBr/Bi2MoO6
BiOBr
0.021
0.026
0.023
0.024
—
2.1 : 0.077 : 1.0
2.1 : 0.14 : 1.0
2.2 : 0.22 : 1.0
2.4 : 0.35 : 1.0
—
3.0 : 0.26 : 1.0
3.5 : 0.45 : 1.0
3.6 : 0.64 : 1.0
4.1 : 0.75 : 1.0
—
23
Fig. 2 FESEM (a, b), images of 0.20-BiOBr/Bi2MoO6.
0.314 nm indicative of the (110) facet in the BiOBr crystal21 and
the (131) facet in the Bi2MoO6 crystal,9 respectively, corre-
sponding to the formation of BiOBr–Bi2MoO6 heterojunctions
at the interface. Moreover, the chemical mapping (Fig. S3†)
Fig. 1 XRD patterns of X-BiOBr/Bi2MoO6, BiOBr and Bi2MoO6.
BiOBr (JCPDS 73-2061) appear in the X-BiOBr/Bi2MoO6 samples conrmed the uniform distribution of Bi, Mo, Br and O
with X $ 1.5. No other diffraction peaks could be observed, elements in the microspheres.
implying the formation of a pure BiOBr/Bi2MoO6 composite.
The XPS spectra in Fig. 4 conrmed that the pure Bi2MoO6
The FT-IR spectra in Fig. S2† revealed that all the samples displayed a Bi binding energy around 158.7 and 164.0 eV in Bi
displayed the vibration peaks of the surface-adsorbed water and 4f5/2 and 4f7/2 levels,22,23 while the pure BiOBr showed higher Bi
hydroxyl groups at 3456 and 1625 cmꢀ1, respectively.17,18 Cor- 4f binding energies of around 159.4 and 164.7 eV.24,25 The Bi 4f
responding to the above XRD results, 0.20-BiOBr/Bi2MoO6 dis- binding energy of 0.20-BiOBr/Bi2MoO6 was between that of
played the same absorbance peaks as the pure Bi2MoO6 rather Bi2MoO6 and BiOBr, which implied an electronic interaction in
than the pure BiOBr due to the low content of BiOBr in the the BiOBr/Bi2MoO6 composite. Additionally, 0.20-BiOBr/
composite. The absorbance signals around 445 and 560 cmꢀ1 Bi2MoO6 displayed higher binding energies of both O 1s and Mo
could be assigned to the vibrations from the bending Mo–O 3d compared to pure Bi2MoO6 as well as a lower binding energy
bond of MoO6 and the bending Bi–O bond of (Bi2O2)2+ in of O 1s and Br 3d than that of pure BiOBr. This could be
Bi2MoO6,19 respectively. Meanwhile, the vibration peaks at 842, attributed to the electron transfer from Bi2MoO6 to BiOBr via
797 and 733 cmꢀ1 could be attributed to the asymmetric the strong interaction in the BiOBr/Bi2MoO6 heterojunction,
stretching of MoO6.19 In comparison, 1.5-BiOBr/Bi2MoO6 corresponding to the above HRTEM image. The higher molar
showed a well-resolved peak at 509 cmꢀ1, indicative of the ratio of both Bi/Mo and Br/Mo in the different X-BiOBr/Bi2MoO6
vibration of Bi–O bond in BiOBr.20
samples determined by XPS compared to those by ICP analysis
As shown in Fig. 2, the FESEM images demonstrated that (see Table 1) further suggested the surface enrichment of Bi and
0.20-BiOBr/Bi2MoO6 was present in the uniform ower-like Br atoms, which was obviously due to the coverage of BiOBr on
microspheres with the average diameter around 2.0 mm. The the surface of the Bi2MoO6 microspheres.
outer shell was covered by orderly assembled nanosheets with
The TG analysis (Fig. S4†) revealed that pure Bi2MoO6
an average thickness of about 15 nm. The TEM image displayed showed no signicant weight loss, while the pure BiOBr dis-
a hollow chamber in the BiOBr/Bi2MoO6 microspheres, and the played a signicant weight loss around 510 ꢁC due to the release
average shell thickness was estimated at 500 nm in Fig. 3. The of Br-contained gas via BiOBr oxidation.25 The BiOBr/Bi2MoO6
HRTEM image clearly shows the lattice distances of 0.277 and showed a higher weight loss temperature than the pure BiOBr.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 13498–13504 | 13499

View Article Online
RSC Advances
Paper
This further conrmed the strong BiOBr–Bi2MoO6 interaction,
in which the Bi2MoO6 could stabilize the BiOBr.
All the BiOBr, Bi2MoO6 and X-BiOBr/Bi2MoO6 samples dis-
played typical IV N2 adsorption–desorption isotherms (Fig. S5†),
indicating the presence of a mesoporous structure. As shown in
Table 1, SBET of the X-BiOBr/Bi2MoO6 samples gradually
increased with the enhanced content of BiOBr in the BiOBr/
Bi2MoO6 composite since the SBET of pure BiOBr was higher
than that of pure Bi2MoO6.
Optical properties
The UV-vis DRS spectra (Fig. S6†) revealed that pure Bi2MoO6
displayed stronger absorbance in the wider visible-light region
compared to that of pure BiOBr due to its narrower energy band
gap. The 0.20-BiOBr/Bi2MoO6 displayed slightly enhanced
visible-light absorbance as the BiOBr nanosheets assembled on
the outer shell of Bi2MoO6 microspheres could enhance light
harvesting via the multi-reections. Fig. S7a† shows that 0.20-
BiOBr/Bi2MoO6 displays the highest photocurrent compared to
pure BiOBr, pure Bi2MoO6 and a mechanical mixture of BiOBr
and Bi2MoO6, which could be attributed to both the enhanced
light harvesting and the diminished photoelectron–hole
recombination rate. The PL spectra (Fig. S7b†) clearly
conrmed a lower photoelectron–hole recombination rate in
the X-BiOBr/Bi2MoO6 composites than in pure BiOBr, pure
Bi2MoO6, and in a mechanical mixture of BiOBr and Bi2MoO6,
corresponding to the weaker PL intensities.21,26 These results
demonstrate that the formation of BiOBr–Bi2MoO6 hetero-
junction plays a key role in inhibiting photoelectron–hole
recombination via transferring photoelectrons from the VB of
Bi2MoO6 to the CB of BiOBr.27 The mechanical mixture of BiOBr
and Bi2MoO6 exhibited a higher photoelectron–hole recombi-
nation rate than the BiOBr/Bi2MoO6 composites, obviously due
to the poor interaction between BiOBr and Bi2MoO6, leading to
fewer BiOBr–Bi2MoO6 heterojunctions. Besides, it was found
that the photoelectron–hole recombination rate decreased with
the increase of BiOBr from 0.10-BiOBr/Bi2MoO6 to 0.20-BiOBr/
Bi2MoO6, corresponding to the increase in the number of
BiOBr–Bi2MoO6 heterojunctions. However, a very high BiOBr
content was harmful for inhibiting photoelectron–hole recom-
bination (see 0.50-BiOBr/Bi2MoO6 and 0.33-BiOBr/Bi2MoO6),
possibly due to the formation of BiOBr and Bi2MoO6 in sepa-
rated phases.
Fig. 3 TEM (a) and HRTEM (b) images of 0.20-BiOBr/Bi2MoO6.
Additionally, the at band potentials (E) of BiOBr and
Bi2MoO6 were determined in EIS (see Fig. S8†). According to the
intercept of the linear relationship between CSCꢀ2 and E, the E
ꢀ2
(CSC ¼ 0) of BiOBr and Bi2MoO6 were calculated. As shown in
Table 2, the conduction band edge (ECB) was 0.1 V more negative
28
than the E in n-type BiOBr and Bi2MoO6 semiconductors
Table 2 Band gap energy and position of BiOBr and Bi2MoO6
Sample
E (eV)
ECB (eV)
EVB (eV)
Eg (eV)
Bi2MoO6
BiOBr
ꢀ0.450
ꢀ0.550
ꢀ0.190
2.04
2.57
2.59
2.76
Fig. 4 XPS spectra of 0.20-BiOBr/Bi2MoO6, BiOBr and Bi2MoO6.
13500 | RSC Adv., 2016, 6, 13498–13504
ꢀ0.0900
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
owing to the positive line slope.29 The edge position of the
valence band could also be determined by considering the
energy band gap calculated from ahv ¼ A(hv ꢀ Eg)n, where a, v,
Eg and A refer to the absorption coefficient, the light frequency,
the band gap and a constant, respectively. The values of n for
Bi2MoO6 and BiOBr were 1/2 and 2, respectively.30,31 Therefore,
the energy band structure in the BiOBr/Bi2MoO6 composite is
proposed, see Scheme 1. Clearly, the band edges of BiOBr (EVB
2.57 eV, ECB ¼ ꢀ0.19 eV) and Bi2MoO6 (EVB ¼ 2.04 eV, ECB
¼
¼
ꢀ0.55 eV) were well matched and thus, their heterojunctions
were favourable for the rapid transfer of photoelectrons from
the VB of Bi2MoO6 to the VB of BiOBr, which reduces the
photoelectron–hole recombination rate.
Photocatalytic performance
As shown in Table 1, the BiOBr exhibited much higher
adsorption capacity for RhB than the Bi2MoO6 owing to the
larger SBET. It was interesting to note that the BiOBr exhibited
much higher adsorption capacity per unit SBET than the
Bi2MoO6, possibly due to the different adsorption models for
RhB. Obviously, each BiOBr active site adsorbed more RhB
molecules than each Bi2MoO6 active site, which could account
for the higher adsorption capacity of the X-BiOBr/Bi2MoO6 than
that of pure Bi2MoO6 with almost the same SBET
.
The visible-light-induced photocatalytic RhB degradation
yields and TOC removal rates on different photocatalysts are
summarized in Fig. 5. Although the BiOBr showed a much
higher adsorption capacity than of the Bi2MoO6 (see Table 1), it Fig. 5 RhB degradation (a) and TOC removal process (b) vs. reaction
time during the visible-light-induced photocatalytic degradation of
still exhibited lower activity, which could be attributed to the
broad energy band gap, corresponding to a poor utilization of
visible light in comparison with the Bi2MoO6. The 0.20-BiOBr/
Bi2MoO6 exhibited much higher activity than those of pure
BiOBr, pure Bi2MoO6, the BiOBr + Bi2MoO6 mechanical mixture
and 0.20-BiOBr/Bi2MoO6(G), in which 0.1 g of 0.20-BiOBr/
RhB on different photocatalysts. Reaction conditions: (a) 100 mL 20
mg Lꢀ1 RhB aqueous solution, 0.10 g ꢀc1atalyst, 300 W Xe lamp (l > 420
nm), T ¼ 30 ꢁC; (b) 100 mL 50 mg L RhB aqueous solution, 0.10 g
catalyst, 300 W Xe lamp (l > 420 nm), T ¼ 30 ꢁC.
Bi2MoO6 was ground for 120 min in the mortar, which damaged
could be mainly attributed to both the enhanced light harvest-
the ower-like structure of 0.20-BiOBr/Bi2MoO6 (see SEM
ing due to the multiple reections in the unique ower-like
images in Fig. S9†). Taking into account the similar adsorption
structure and the diminished photoelectron–hole recombina-
capacities for RhB, the high activity of 0.20-BiOBr/Bi2MoO6
tion rate owing to the formation of BiOBr–Bi2MoO6 hetero-
junctions resulting from their strong interaction.32–34 The low
activities of pure BiOBr and Bi2MoO6 were due to the easy
recombination of photo-induced holes and electrons, and the
slightly increased activity of the BiOBr + Bi2MoO6 sample may
be due to the random and unstable contact between Bi2MoO6
and BiOBr in the physical mixture, which is favourable for the
charge separation to some extent. However, it is impossible to
form stable BiOBr–Bi2MoO6 heterojunctions to greatly improve
the photocatalytic activity. The 0.20-BiOBr/Bi2MoO6(G) sample
exhibited lower activity than 0.20-BiOBr/Bi2MoO6 due to the
damage to the ower-like structure (see Fig. S9†). The TOC
measurement showed the change in the total amount of
organics, including the original RhB and the intermediates
during the photocatalytic process. Both the original RhB and
the intermediates were photodegraded simultaneously. There-
fore, the decreased TOC value was obviously lower than that of
the RhB degradation ratio together with a certain difference in
Scheme 1 Illustration of the energy band structure and photocatalytic
reaction process of the BiOBr/Bi2MoO6 heterojunction.
activity (Fig. 5b), and RhB was photodegraded to CO2. By
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 13498–13504 | 13501

View Article Online
RSC Advances
Paper
plotting (C0 ꢀ C)/C vs. reaction time, linear plots were obtained
for all of the prepared photocatalysts (see Fig. S10†), which
indicated that the RhB degradation reaction was a zero-order
reaction with respect to RhB concentration. 0.22-BiOBr/
Bi2MoO6 showed the very highest kinetic constants due to the
highest photocatalytic efficiency. Similar results were also ob-
tained during visible-light-induced photocatalytic degradation
of methyl orange (MO) and phenol (Fig. S11†). It was also found
that in the X-BiOBr/Bi2MoO6 series, their activities rst
increased with the increasing BiOBr content from pure Bi2MoO6
to 0.10-BiOBr/Bi2MoO6 to 0.20-BiOBr/Bi2MoO6 and then
decreased with the further increase of BiOBr content from 0.20-
BiOBr/Bi2MoO6 to 0.33-BiOBr/Bi2MoO6 to 0.50-BiOBr/Bi2MoO6.
The 0.20-BiOBr/Bi2MoO6 sample displayed the maximum
activity (Fig. S12†). Such results were exactly the same as the
changing order of PL intensities (Fig. S7†), suggesting that the
photoelectron–hole recombination rate plays a crucial role in
determining photocatalytic activity. The insufficient BiOBr–
Bi2MoO6 heterojunctions in the 0.10-BiOBr/Bi2MoO6 sample
resulted in an inefficient inhibition of charge recombination,
while more BiOBr catalyst in both the 0.33-BiOBr/Bi2MoO6 and
Fig. 6 Effects of various scavengers on the photocatalytic efficiencies
of 0.20-BiOBr/Bi2MoO6 in RhB photocatalytic degradation. Reaction
conditions are given in Fig. 5a.
the CB of Bi2MoO6 were injected into the CB of BiOBr via their
heterojunctions, followed by reacting with the adsorbed O2 to
produce cO2ꢀ, owing to the higher redox potential of O2/cO2ꢀ (Eo
¼ ꢀ0.046 eV)45 than the ECB of BiOBr. Finally, cO2ꢀ and the main
active species of h+ led to the oxidation of RhB.
0.50-BiOBr/Bi2MoO6
recombination.
samples
induced
easier
charge
In order to examine the formation of cOH radicals during the
photodegradation reaction, terephthalic acid photo-
a
In the photodecomposition process in dye wastewater
treatment, it is important to evaluate the mineralization ability
of catalysts since the yielded intermediates may be more toxic
than the dye molecule itself. Since the multiple yielded inter-
mediates may also exhibit activity on the visible-light-driven
RhB degradation process,33 the photocatalytic RhB degrada-
tion yields and TOC removal rates under visible lights show
certain difference of activity in Fig. 5a and b. Hence, the yielded
intermediates of the visible-light-driven RhB degradation
process on 0.20-BiOBr/Bi2MoO6 were further investigated by
HPLC-MS (Fig. S14†). Moreover, the N,N,N0,N'-tetraethylrhod-
amine (RhB) molecule with an m/z of 443, ve intermediates
with m/z of 415, 387a, 387b, 359 and 331 were detected, corre-
sponding to N,N,N0-triethylrhodamine (TER), N,N0-diethylrhod-
luminescence (TA-PL) probing technique35,36 with an excitation
wavelength of 315 nm was employed. In Fig. S13,† PL signals
were observed at 425 nm with the irradiation time (excitation at
315 nm). This demonstrated that cOH radicals were formed
during the photodegradation process. However, as shown in
Fig. S13,† 0.20-BiOBr/Bi2MoO6 exhibited similar TA-PL peak
intensity at 425 nm compared to Bi2MoO6, but much lower than
BiOBr, implying that cOH radicals were mainly generated on
BiOBr in the BiOBr/Bi2MoO6 composite. It could be ascribed
that only the photo-induced holes on the BiOBr photocatalyst
rather than that on Bi2MoO6 were able to oxidize either H2O or
OHꢀ directly to cOH, since both redox potentials of cOH/H2O (Eo
¼ 2.38 V)37 and cOH/OHꢀ (Eo ¼ 2.27 V)38,39 were more positive
than the EVB of Bi2MoO6, but were more negative than the EVB of
BiOBr. Therefore, it was difficult to oxidize H2O or OHꢀ into
cOH in the Bi2MoO6/BiOBr heterojunction due to the low
content of BiOBr. Additionally, Fig. 6 reveals that the addition of
isopropanol (IPA) allowed it to act as an cOH scavenger,40,41 thus
displaying no signicant inhibiting effect on the photocatalytic
RhB degradation. However, the addition of ammonium oxalate
(AO) as h+ (photogenerated holes) scavengers42 could almost
completely suppress the photocatalytic RhB degradation. The
amine
(DERa),
N,N-diethylrhodamine
(DERb),
N-
ethylrhodamine (MER) and rhodamine. The intermediates were
not detected, as they were more toxic than the dye molecule
itself. The RhB concentration decreased monologically with the
increasing reaction time, while the concentrations of all the
intermediates rst increased and then decreased. According to
the appearance order of multiple intermediates, we concluded
that the RhB degradation followed the de-ethylation route and
cleaved into TER, DER, MER and rhodamine in turn, and nally
decomposed into CO2.
ꢀ
addition of benzoquinone (BQ) as an cO2 scavenger40,43 and
AgNO3 as an eꢀ scavenger44 also resulted in decreased activity,
respectively. This demonstrated that h+ was the main active site
and could oxidize the RhB directly rather than rst following the
process of forming cOH.
Conclusions
According to the abovementioned results, the plausible This work developed a facile solvothermal approach to prepare
mechanism of RhB photocatalytic degradation on the BiOBr/ BiOBr/Bi2MoO6 composite photocatalyst in ower-like
a
Bi2MoO6 composite photocatalyst is proposed in Scheme 1. microspheres. It exhibited high activity in the photocatalytic
Under visible-light irradiation, the electrons could be excited degradation of organic pollutants under visible-light irradia-
from the VB of Bi2MoO6 and BiOBr to their CB. The electrons in tion, mainly owing to enhanced light harvesting via multi-
13502 | RSC Adv., 2016, 6, 13498–13504
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
reections and reduced photoelectron–hole recombination were maintained at 60ꢁ to the detection plane, which was
through the heterojunction. Kinetic studies demonstrated that dened by the Xe lamp, the samples and the detector. Mean-
the RhB degradation to CO2 were mainly driven by h+ active while, cOH-trapping photoluminescence spectra of terephthalic
sites and followed a de-ethylation route with the formation of acid solution (PL-TA, lexcition ¼ 315 nm, lemission ¼ 425 nm) were
ve intermediates.
recorded on a photoluminescence spectrum (PL, Varian Cary-
Eclipse 500). The metal ion concentrations were determined
by using inductively coupled plasmatomic emission spectros-
copy (ICP, VISTA-MPX). Photocurrent-time measurements and
electrochemical impedance spectroscopy (EIS) were performed
Experimental section
BiOBr/Bi2MoO6 preparation
using
an
electrochemistry
workstation
(CHI660E).
3.4 g Bi(NO3)3$5H2O and 0.84 g Na2MoO4$2H2O were dissolved
in 10 mL ethylene glycol (EG) and 10 mL glycerol under stirring
for 120 min, respectively. Aer mixing the two solutions, 40 mL
ethanol was slowly added, followed by stirring for 10 min. The
clear solution was then transferred into a 100 mL Teon-lined
autoclave and heated at 160 ꢁC for 24 h. The brown suspen-
sion with Bi2MoO6 precursor was obtained aer the autoclave
was cooled to room temperature naturally. Then, the desired
amounts of Bi(NO3)3$5H2O and cetyltrimethyl ammonium
bromide (CTAB) with a 1.5 molar ratio to Bi(NO3)3$5H2O was
added with stirring for 120 min. The solution was transferred
into a 100 mL Teon-lined autoclave and heated at 160 ꢁC for 10
h again. Finally, the obtained solid was ltered, washed with
ethanol, dried at 80 ꢁC in air and annealed at 400 ꢁC for 4 h. The
as-prepared sample was denoted as X-BiOBr/Bi2MoO6, where X
refers to the Br/Mo molar ratio. For comparison, the pure
Bi2MoO6 and BiOBr samples were prepared by the same
procedures, respectively. The mechanical mixture of BiOBr and
Bi2MoO6 was referred to BiOBr + Bi2MoO6, in which the Br/Mo
molar ratio was the same as that in 0.20-BiOBr/Bi2MoO6. The
0.20-BiOBr/Bi2MoO6(G) sample was 0.20-BiOBr/Bi2MoO6 aer
being crushed.
Photocurrent-time measurements were carried out on the lm
sample in 0.10 M Na2SO4 aqueous solution with an exposed
surface area of 2.0 cm2, which was irradiated by a 300 W Xe
lamp (l > 420 nm). EIS Experiments were carried out on the lm
sample in 0.10 M Na2SO4 solution at 1000 Hz within the
potential region of ꢀ1.0 to 1.0 V vs. Ag/AgCl. A Mott–Schottky
2ꢀ
(M–S) plot showed the linear variation of CSC with respect to
the applied potential (eqn (1)), where ND, 3, E, E and CSC refer
to the doping density, dielectric constant, at band potential,
external biasing voltage and capacitance, respectively.
ꢀ2
CSC ¼ (1.41 ꢂ 1020/ND3)[E ꢀ Efb ꢀ 0.026]
(1)
Activity test
The liquid-phase photocatalytic degradation of dyes was carried
out at 30 C in a self-designed 250 mL reactor containing 0.10 g
ꢁ
catalyst and 100 mL 20 mg Lꢀ1 dye organics. A 300 W Xe lamp was
used as a visible-light source, and a 420 nm lter was placed above
the reactor to cut off the UV light. At given time intervals, 3.0 mL
aliquots were collected from the suspension and immediately
centrifuged. The concentration of dyes was determined using a UV-
vis spectrophotometer (UV-7504 PC) at the characteristic peak. A
liquid chromatography-mass spectrometry system (HPLC-MS,
Agilent 1200) was used to detect the reaction products and inter-
mediates. The measurement of total organic carbon (TOC) was
carried out on a Vario TOC analyzer. The reproducibility of the
results was checked by repeating the experiments at least three
times and was found to be within acceptable limits (ꢃ6%).
Catalyst characterization
The structure and composition were determined by energy
dispersive X-ray spectroscopy (EDX, HITACHI S-4800) and X-ray
diffraction (XRD, Rigacu D/Max-2000). Surface morphologies
were observed by scanning electron microscopy (FESEM,
HITACHI S-4800) and by using a transmission electronic
micrograph (TEM, JEM-2010). The thermal stability was inves-
tigated by thermogravimetric analysis (TGA) on a DTG-60H. The
FTIR spectra were recorded by a Fourier transform infrared
spectroscopy (NEXUS 470). X-ray photoelectron spectroscopy
(XPS, Versa Probe PHI 5000) was employed to determine the
surface electronic states. The shi of the binding energy due to
the relative surface charging was corrected using the C1S level at
284.6 eV as an internal standard. N2 adsorption/desorption
isotherms were measured on a TriStar II 3020. The Brunauer–
Emmett–Teller (BET) method was used to calculate the specic
surface area (SBET) based on the adsorption branches. The
optical properties were determined by UV-vis diffuse reectance
spectrum (UV-vis DRS, Varian Cary 500) and photo-
luminescence spectrum (PL, Varian Cary-Eclipse 500). The
excitation wavelength was 325 nm provided by a UV xenon lamp
with an incidence angle of 45ꢁ, and the spectra were recorded in
the range from 400 to 600 nm at room temperature. To avoid
reecting the excitation light into the detector, all the samples
Acknowledgements
This work is supported by National Natural Science Foundation
of China (21237003 and 21577092), Program for Changjiang
Scholars and Innovative Research Team in University (IRT1269),
Shanghai Government (15520711300) International Joint
Laboratory on Resource Chemistry (IJLRC), Science and tech-
nology project of Yunnan Province (C0120150543) and Research
Fund of Yunnan Provincial Education Department (2015Z183).
Notes and references
1 W. Z. Yin, W. Z. Wang and S. M. Sun, Catal. Commun., 2010,
11, 647–650.
2 A. Martinez-de Cruz and S. Obregon Alfaro, J. Mol. Catal. A:
Chem., 2010, 320, 85–91.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 13498–13504 | 13503

View Article Online
RSC Advances
Paper
´
´
3 E. L. Cuellar, A. M. Cruz, K. H. L. Rodrıguez and 25 Y. C. Feng, L. Li, J. W. Li, J. F. Wang and L. Liu, J. Hazard.
´
U. O. Mendez, Catal. Today, 2011, 166, 140–145.
4 L. W. Zhang, T. G. Xu, X. Zhao and Y. F. Zhu, Appl. Catal., B, 26 J. C. Yu, J. G. Yu, W. K. Ho, Z. T. Jiang and L. Z. Zhang, Chem.
2010, 98, 138–146. Mater., 2002, 14, 3808–3816.
5 X. Lin, X. Y. Guo, W. L. Shi, H. J. Zhai, Y. S. Yan and 27 J. Zhang, J. X. Xia, S. Yin, H. M. Li, H. Xu, M. Q. He,
Q. W. Wang, J. Solid State Chem., 2015, 229, 68–77. L. Y. Huang and Q. Zhang, Colloids Surf., A, 2013, 420, 89–95.
6 J. Di, J. X. Xia, M. X. Ji, H. P. Li, H. Xu, H. M. Li and R. Chen, 28 S. R. Morrison, Electrochemistry at Semiconductor and
Mater., 2011, 92, 538–544.
Nanoscale, 2015, 7, 11433–11443.
Oxidized Metal Electrodes, Plenum Press, New York, 1980,
ch. 4.
29 J. M. Herrmann, M. E. Jamal and M. Forissier, React. Kinet.
Catal. Lett., 1988, 37, 255–260.
30 M. Y. Zhang, C. L. Shao, P. Zhang, C. Y. Su, X. Zhang and
P. P. Liang, J. Hazard. Mater., 2012, 225–226, 155–163.
7 J. Ren, W. Z. Wang, M. Shang, S. M. Sun and E. P. Gao, ACS
Appl. Mater. Interfaces, 2011, 3, 2529–2533.
8 Y. S. Xu, Z. J. Zhang and W. D. Zhang, Mater. Res. Bull., 2013,
48, 1420–1427.
9 P. Zhang, C. L. Shao, M. Y. Zhang, Z. C. Guo, J. B. Mu,
Z. Y. Zhang, X. Zhang and Y. C. Liu, J. Hazard. Mater., 31 X. J. Shi, X. Chen, X. L. Chen, S. M. Zhou, S. Y. Lou,
2012, 217–218, 422–428. Y. Q. Wang and L. Yuan, Chem. Eng. J., 2013, 222, 120–127.
10 F. J. Zhang, S. F. Zhu, F. Z. Xie, J. Zhang and Z. D. Meng, Sep. 32 J. Cao, B. Y. Xu, H. L. Lin, B. D. Luo and S. F. Chen, Chem.
Purif. Technol., 2013, 113, 1–8. Eng. J., 2012, 185–186, 91–99.
11 N. Li, L. Zhu, W. D. Zhang, Y. X. Yu, W. H. Zhang and 33 Y. C. Miao, G. F. Pan, Y. N. Huo and H. X. Li, Dyes Pigm.,
M. F. Hou, J. Alloys Compd., 2011, 509, 9770–9775. 2013, 99, 382–389.
12 Y. S. Xu and W. D. Zhang, Appl. Catal., B, 2013, 140–141, 306– 34 H. X. Li, Z. F. Bian, J. Zhu, D. Q. Zhang, G. S. Li, Y. N. Huo,
316. H. Li and Y. F. Lu, J. Am. Chem. Soc., 2007, 129, 8406–8407.
13 F. Zhou, R. Shi and Y. F. Zhu, J. Mol. Catal. A: Chem., 2011, 35 K. Ishibashi, A. Fujishima, T. Watanabe and K. Hashimoto,
340, 77–82. Electrochem. Commun., 2000, 2, 207–210.
14 X. Zhao, H. J. Liu, Y. L. Shen and J. H. Qu, Appl. Catal., B, 36 Q. Xiao, Z. C. Si, J. Zhang, C. Xiao and X. K. Tan, J. Hazard.
2011, 1–2, 63–68. Mater., 2008, 150, 62–67.
15 H. Deng, J. W. Wang, Q. Peng, X. Wang and Y. D. Li, Chem.– 37 A. J. Bard and L. R. Faulkner, Electrochemical Methods:
Eur. J., 2005, 11, 6519–6524.
Fundamentals and Applications, John Wiley & Sons, New
16 W. L. Huang and Q. S. Zhu, Comput. Mater. Sci., 2008, 43,
1101–1108.
York, 1980.
38 P. L. Allen and A. Hickling, Trans. Faraday Soc., 1967, 53,
17 J. C. Yu, L. Z. Zhang, Z. Zheng and J. Zhao, Chem. Mater.,
2003, 15, 2280–2286.
1626–1635.
39 X. Li, J. Zhu and H. X. Li, Appl. Catal., B, 2012, 123–124, 174–
18 T. Y. Peng, D. Zhao, H. B. Song and C. H. Yan, J. Mol. Catal. A:
Chem., 2005, 238, 119–126.
19 M. Maczka, L. Macalik, K. Hermanowicz, L. Kepinski and
J. Hanuza, J. Raman Spectrosc., 2010, 41, 1289–1296.
181.
40 G. T. Li, K. H. Wong, X. W. Zhang, C. Hu, J. C. Yu,
R. C. Y. Chan and P. K. Wong, Chemosphere, 2009, 76,
1185–1191.
20 Z. S. Liu, Y. H. Bi, Y. L. Zhao, X. Huang and Y. B. Zhu, Mater. 41 L. S. Zhang, K. H. Wong, H. Y. Yip, C. Hu, J. C. Yu, C. Y. Chan
Res. Bull., 2014, 49, 167–171. and P. K. Wong, Environ. Sci. Technol., 2010, 44, 1392–1398.
21 D. Q. Zhang, M. C. Wen, B. Jiang, G. S. Li and J. C. Yu, J. 42 N. Zhang, S. Q. Liu, X. Z. Fu and Y. J. Xu, J. Phys. Chem. C,
Hazard. Mater., 2012, 211–212, 104–111. 2011, 115, 9136–9145.
22 X. Zhao, J. H. Qu, H. J. Liu and C. Hu, Environ. Sci. Technol., 43 M. C. Yin, Z. S. Li, J. H. Kou and Z. G. Zou, Environ. Sci.
2007, 41, 6802–6807. Technol., 2009, 43, 8361–8366.
23 G. P. Dai, J. G. Yu and G. Liu, J. Phys. Chem. C, 2011, 115, 44 Y. N. Huo, J. Zhang, M. Miao and Y. Jin, Appl. Catal., B, 2012,
7339–7346. 111–112, 334–341.
24 X. M. Tu, S. L. Luo, G. X. Chen and J. H. Li, Chem.–Eur. J., 45 L. Kong, Z. Jiang, H. H. Lai, R. J. Nicholls, T. C. Xiao,
2012, 18, 14359–14366.
M. O. Jones and P. P. Edwards, J. Catal., 2012, 293, 116–125.
13504 | RSC Adv., 2016, 6, 13498–13504
This journal is © The Royal Society of Chemistry 2016